Differential corrections for location information provided by a global positioning system (GPS) offer the possibility of reducing the inaccuracies in location and observation time provided by such system from tens of meters and tens of nanoseconds to one meter or less and one nanosecond or less, respectively. However, differential GPS (DGPS) corrections for determinations of location and/or time are usually limited to a region no more than a few hundred kilometers (km) in diameter. For regions of larger diameter, errors due to differences in ionospheric delay, tropospheric delay, presence of multipath signals and similar phenomena accumulate and can increase the total location error of a DGPS calculation to tens of meters.
Carrier phase measurements using the Navstar Global Positioning System (GPS) allow the measurement of short baselines (&lt;300 kilometers) with an inaccuracy as low as a few centimeters. Originally limited to static situations with the signal antennas remaining stationary over a time interval as long as one hour, this technology has been extended to allow one antenna to move relative to another antenna (kinematic survey mode) over baselines as short as 20-30 km. More recently, real time location determination systems have appeared that allow such baseline measurements in the field. This extends carrier phase applications from traditional mapping and control survey into new positioning and navigation applications with centimeter-level accuracy. These new systems are especially sensitive to small biases when recovering from carrier lock loss, which might occur when the GPS antenna moves under a tree or highway overpass or past a tall building. For this reason, these new systems are limited to relatively short baselines, to prevent the systematic biases that accumulate over distance from affecting the reacquisition process, including phase integer ambiguity resolution after carrier lock loss.
Phase integer ambiguity resolution techniques use interferometric approaches. Three resolution techniques are widely used with GPS. Two of these techniques, code averaging and kinematic Doppler analysis, produce location errors that are linear with respect to small bias errors. See U.S. Pat. Nos. 4,812,991 and 4,963,889, issued to R. Hatch, and G. Seeber and O. Wubbena, "Kinematic Positioning With Carrier Phases And `On The Way` Ambiguity Solution", Proc. Fifth Intl. Geodetic Symposium on Satellite Positioning, Albuquerque, N. Mex., Mar. 13-17, 1989, pp. 600-609, for a discussion of the code averaging technique. See P. Loomis, "A Kinematic GPS Double Differencing Algorithm", Proc. Fifth Intl. Geodetic Symposium on Satellite Positioning, Albuquerque, N. Mex., Mar. 13-17, 1989, pp. 610-620, and P. Hwang, "Kinematic GPS for Differential Positioning: Resolving Integer Ambiguities on the Fly", Navigation, vol. 38, pp. 1-15, for a discussion of kinematic Doppler analysis.
A third technique, referred to here as the Maximum Likelihood Integer Ambiguity Resolution (MLIAR) technique is potentially the most powerful and efficient, but this technique is nonlinear with respect to small bias errors in the input signals. An MLIAR solution may show no perceptible response to inclusion of small bias errors until these errors grow to a threshold size, at which point the integer solution chosen becomes a false set of integers and the phase solution may change by many wavelengths (tens of decimeters).
The current approach to limiting the effect of bias errors in the MLIAR technique is to keep the baselines relatively short, no more than 30 km, so that bias errors are no more than 1 mm per km of baseline length. The U.S. Department of Defense proposes to degrade the accuracy by disguising the true orbits, thereby imposing Selective Availability on the clock and orbit information received from the satellites. This would increase the bias errors about ten-fold and would reduce the maximum acceptable baseline lengths for MLIAR by a similar factor.
Several workers have disclosed time distribution systems using a master or base station and one or more subsidiary time signal receivers. An example is U.S. Pat. No. 3,520,128, issued to Novikov et al. An independent primary clock is connected to, and provides exact time signals for, a plurality of secondary clocks by radio waves. Each secondary clock receives a sequence of uncorrected "exact" time signals and a sequence of timing marks to correct this uncorrected time. The time signals for each secondary clock are apparently corrected separately.
Cater, in U.S. Pat. No. 3,811,265, discloses transmission of coded, time-indicating signals from a master clock at a central station to one or more slave clocks, using a two-wire line and binary-valued pulses with different time durations. A time synchronizing pulse is periodically inserted (e.g., once per second) on the line to correct for drift or other errors. If the two-wire line is a standard 60-cycle power line or a television cable, the binary-valued pulses use one or more frequencies that lie outside the frequency range normally used on that line, to avoid signal interference with the standard signals transmitted over that line.
A clock that can be synchronized by "wireless" signals is disclosed by Gerum et al in U.S. Pat. No. 3,881,310. The clock contains an electromagnetically operated mechanical oscillator whose frequency 2f0 is twice the rated frequency of an alternating current network connected to the clock. A time synchronization module transmits a signal of frequency f1&gt;&gt;f0 that is modulated by the network at 2f0 and received and demodulated by the clock. Normally, the pulses received from the network drive the clock and the oscillator is in a standby mode. The clock oscillator is enabled, and the network is disconnected, when and only when the network frequency differs by at least a predetermined amount from the frequency 2f0 of the oscillator. The oscillator in standby mode receives resonance energy of frequency.apprxeq.2f0 from the network for maintaining the oscillations.
Chappell et al, in U.S. Pat. No. 3,941,984, discloses a satellite-aided navigation system in which location fixes of a mobile station are made at selected times. Between any two such selected times, the mobile station velocity is determined using Doppler shifts of signals received from the satellites. These velocities, measured at intermediate times, are converted to location coordinate increments and added to the location coordinates given by the last location fix to provide intermediate location coordinates between consecutive location fixes.
Cateora et al, in U.S. Pat. No. 4,014,166, disclose a satellite-controlled digital clock system for maintaining time synchronization. A coded message containing the present time and satellite position is transmitted from a ground station to an orbiting satellite and is relayed to a group of ground-based receivers. A local oscillator aboard the satellite is phase-locked to a precise frequency to provide the system with accurate time-of-year information by a count of the accumulated pulses produced by the oscillator. This count is compared with a time count determined from the coded message received by the satellite. After a selected number of errors are observed through such comparisons, the on-board clock is reset to the time indicated by the coded messages received. If transmission of the coded messages is interrupted, the on-board oscillator continues to provide time information that is transmitted to the ground-based receivers.
U.S. Pat. No. 4,042,923, issued to Merrick, discloses a trilateralization method for determination of location coordinates of a mobile station. Two stationary transceivers, each having a known location and being spaced apart, each transmit a stream of timed radar pulses having a unique code embedded therein, and these two streams are received by the mobile station. The mobile station fixes and stores its present location by determining the average distance between itself and each of the transceivers in a particular time interval of length approximately 1-1000 .mu.sec, using triangulation. If the changes of the mobile station location coordinates are not within reasonable limits, the location fix is rejected and the last valid location fix is used.
An antenna space diversity system for TDMA communication with a satellite is disclosed by U.S. Pat. No. 4,218,654, issued to Ogawa et al. Differences of temporal lengths of paths from the satellite through each antenna to a ground-based signal processor station are determined by measurement of times required for receipt of pre-transmission bursts sent in the respective allocated time slots through two different antennas, in a round trip from base station to satellite to base station. Variable time delays are then inserted in the base station signal processing circuits to compensate for the temporal length differences for the different signal paths. These time delays are changed as the satellite position changes relative to each of the antennas.
U.S. Pat. No. 4,287,597, issued to Paynter et al, discloses receipt of coded time and date signal from two geosynchronous satellites, which signals are then converted into local date and time and displayed. The frequency spectrum is scanned by an antenna to identify and receive the satellite signals. Temporal length differences for signal paths from each satellite through a receiving antenna to a signal processing base station are determined, to provide compensation at the base station for these differences. Time information is provided by a satellite every 0.5 seconds, and this information is corrected every 30 seconds. Signals from either or both satellites are used to provide the time and date information, in normal local time and/or daylight savings local time.
Jueneman discloses an open loop TDMA communications system for spacecraft in U.S. Pat. No. 4,292,683. A spacecraft, such as a satellite, in quasi-geosynchronous orbit carries a transponder that relays a coded signal from a ground-based signal-transmitting station to a plurality of spaced apart, ground-based receivers. This coded signal includes a time index and an index indicating the spacecraft's present position. The time index is adjusted by each receiver to compensate for the changing position of the spacecraft through which the coded signal is relayed. The system is open loop and requires no feedback from the receivers to the base station.
Nard et al, in U.S. Pat. No. 4,334,314, disclose a system for radio wave transmission of time-referenced signals between two ground-based stations, with compensation for multi-path transmission timing errors. Station no. 1 has a single antenna. Station no. 2 has two antennas, spaced apart by a selected distance, to allow measurement of and compensation for multi-path transmission path length differences. A signal processor located at the receiver antenna combines a plurality of timing marks, received from the transmitting antenna along multiple paths, into a single timing mark that compensates for the multiple path length differences. This arrangement allegedly allows station-to-station transmission over distances as large as ten times the trans-horizon or direct sighting distance (which is approximately proportional to the square root of the product of antenna height and Earth's radius).
U.S. Pat. No. 4,337,463, issued to Vangen, discloses time synchronization between a master station and a remote station in which a coded message, transmitted by the master station, is received by and activates a counter in, the remote station. The remote station adds to the time value contained in the coded message the length of the message as determined by the counter and replaces the old time value by this sum. In this manner, the master and remote stations can be time synchronized.
Method and apparatus for determining the elapsed time between an initiating event and some other event are disclosed by U.S. Pat. No. 4,449,830, issued to Bulgier. A first timer and a second time mark the times of occurrence, respectively, of an initiating event and a subsequent event that depends upon occurrence of the initiating event. The two timers are initially connected and synchronized, then disconnected before the initiating event occurs. The timers are then reconnected after both events have occurred, to allow determination of the elapsed time between occurrence of the two events.
Distance ranging and time synchronization between a pair of satellites is disclosed by Schwartz in U.S. Pat. No. 4,494,211. Each satellite transmits a timing signal and receives a timing signal from the other satellite. The difference in time, including compensation for signal processing delay on a satellite, between transmission and receipt of the signals is transmitted by each satellite to the other satellite and is used to establish time synchronization and to determine the distance between the two satellites. This exchange of signals would be repeated at selected time intervals to maintain synchronization, where the satellites are moving relative to each other. No communications link to a third entity is required, and only one of the satellite clocks need be adjusted to establish and maintain time synchronization.
In U.S. Pat. No. 4,543,657, Wilkinson discloses a system for synchronizing two clocks by transmission of a single pseudo-random number (PRN) that is an unbroken stream of bits by a master clock. At any point in time, the accumulated partial bit stream represents a unique time of day. One bit of the number is transmitted every 10 msec, and the bit stream recycles every 24 hours. The total coded signal for 24 hours is thus 8,640,000 bits long, and the time resolution is .+-.5 msec. The partial bit stream is received and decoded by a receiver and applied to synchronize a remote clock associated with this receiver. In order to compensate for signal propagation time between the clocks, it appears that the distance between the master and remote clocks would have to be known and fixed. An earlier U.S. Pat. No. 3,852,534, issued to Tilk, discloses a method for maintaining synchronization between two pseudorandom number generators at spaced apart locations, using a common time generating source. The times for signal propagation between the two generators may vary.
Plangger et al, in U.S. Pat. No. 4,582,434, disclose transmission and receipt of a continuously corrected single sequence of timing signals. A microprocessor at the receiver periodically compares these timing signals with on-board timing signals generated by a local clock. A varactor diode in a crystal oscillator circuit is adjusted to adjust the microprocessor's operating frequency to minimize any error between the two timing signal sequences. Timing signal processing delay time is compensated for in a receiver circuit. The frequency for microprocessor operation is continuously corrected. If the transmitted timing signals are too weak or do not arrive, the on-board timing signals control the microprocessor until the transmitted timing signals are received in sufficient strength again.
A remote time calibration system using a satellite is disclosed in U.S. Pat. No. 4,607,257, issued to Noguchi. A base station provides a reference system of absolute timing signals and transmits these to a satellite that orbits the Earth. The satellite then calibrates and periodically adjusts its internally generated time and transmits observed data plus the corresponding adjusted satellite time to one or more data receiving stations on the Earth that are distinct from the base station. Time calibration optionally compensates for signal propagation time delay from base station to satellite and allows continuous transmission of data from satellite to the data receiving station(s). Several time difference indicia are computed here.
In U.S. Pat. No. 4,809,005, Counselman discloses a system for accurately determining the location of a water-borne vessel, using four or more intercommunicating, land-based GPS stations with fixed locations, where a designated GPS station collects and communicates GPS station information to the vessel. This configuration is also used to measure the vessel location relative to the location of the designated GPS station.
Olsen et al, in U.S. Pat. No. 4,814,711, disclose a real time geophysical survey system including four or more GPS satellites, a fixed base station on the ground, and one or more GPS data acquisition vehicles that communicate with the central station and that receive GPS signals from the satellites. Each vehicle determines its approximate horizontal location, and the central station transmits signals to guide each vehicle along a selected pattern for purposes of collecting survey data. Periodically, the survey data are transmitted by each vehicle to the central station for analysis, display and/or storage, using time-synchronized signals.
A system for obtaining orbital data from GPS satellites is disclosed by Counselman, in U.S. Pat. No. 4,912,475. Satellite signals are received by three or more spaced apart, fixed ground stations that form a network of baselines between these stations. The ratio of maximum baseline length to minimum baseline length is much greater than 1. From the satellite signals received at each pair of base stations, certain double-differenced carrier phase measurements are formed and formed and used to resolve phase integer ambiguities associated with carrier phase signals received at the base stations.
Hatch discloses a method for resolving the integer ambiguities associated with use of GPS carrier phase data in U.S. Pat. No. 4,963,889, using a reference receiver, a roving receiver and the L1 and L2 carrier phase information received from three or more GPS satellites. The procedure identifies a large group of candidate integer solutions and then successively eliminates most of these candidates by use of further constraints.
U.S. Pat. No. 5,099,245, issued to Sagey, discloses an airborne vehicle location system that uses one or more satellites and three or more spaced apart base stations with fixed, known locations on the ground to determine the location of the vehicle. Signals are transmitted by the vehicle to the base stations, to the satellite, and relayed by the satellite (acting as a "bent pipe" transponder) to a separate central station. The distance of each base station from the airborne vehicle is determined by relative time delays for arrival of these signals at the central station, and these distances determine the location of the airborne vehicle.
A system for determining the location of a mobile station, using two or more signal relay satellites with known locations and a base station on the ground is disclosed by Toriyama, in U.S. Pat. No. 5,111,209. The mobile station transmits an initiating signal to a first satellite, and this signal is relayed to the base station. The signal is than transmitted simultaneously to the first and second satellites, which relay this signal to the mobile station. The location of the mobile station is determined by the relative times at which these relayed signals from the two satellites are received by the mobile station.
Allison, in U.S. Pat. No. 5,148,179, discloses a method for obtaining differential corrections to GPS location determination, using carrier phase information. Pseudorange double differences involving a reference receiver, a roving receiver, a pivotal satellite and three other satellites are formed and used to estimate or resolve the integer ambiguities associated with carrier phase data. The method can be used with a linear combination of the L1 and L2 GPS carrier signals or where only one of these two signals is available.
U.S. Pat. No. 5,155,490, issued to Spradley, discloses a geodetic survey system using four or more GPS satellites and three or more GPS base stations with fixed, known locations on the ground. Clock drift and clock offset for a clock in each of the network of base stations is determined and compensated for by time averaging. A mobile station receives location-determining signals from each of the network of base stations and thereby determines its location, by post-processing or by activities that approximate real time processing.
In U.S. Pat. No. 5,138,631, Taylor discloses a satellite communication network having a central station and a plurality of subsidiary stations, all ground-based. An inbound message from a satellite carries packets encoded in CDMA code, indicating the intended destination. The central station receives these messages and transmits the messages to the appropriate subsidiary stations, using TDMA coding, at a faster bit rate. The central station may serve several groups of subsidiary stations, each using a different CDMA code.
Penrod, in U.S. Pat. No. 5,220,333, discloses a method for synchronizing a given clock time to Universal Coordinated Time (UTC) from a sequence of LORAN-C signal transmissions. The time must be initially accurate to within 100 msec of UTC, and the location of the clock must be initially known to within 5 miles. The system then resynchronizes this time to UTC, accurate to within a few msec, by identifying the correct LORAN-C time cycle and compensating approximately for receiver time delay and signal propagation time delay.
The approaches disclosed in these patents generally: (1) assume that the location of each timing signal recipient is known precisely, or (2) usually provide timing signals and/or location signals that are, at best, accurate to within a few nanoseconds or (equivalently) to within a few meters or (3) do not provide real-time signals with high accuracy for timing synchronization or location determination. What is needed is a system that provides real-time or near-real-time signals to support radio-based location determination, where the system has associated inaccuracies of no more than a few centimeters and provides the particular accuracy required for resolving integer ambiguities. Preferably, the system should reduce the biases that currently limit the usefulness of the MLIAR technique and should reduce or eliminate the additional bias intentionally introduced by the U.S. Department of Defense.
Some systems are currently available that perform this function over distances of the order of 10-20 kilometers (km). An example of such a current system is the Hatch patent, U.S. Pat. No. 4,963,889, discussed above. However, the accuracy of such a system quickly degrades with distance so that integer ambiguity resolution techniques cannot be used distances of the order of 100 km or more. To be practical, a service area for a location determination system should extend over a 100-km broadcast radius for a typical tower-based, line-of-sight radio transmitter. Preferably, the system should provide correction messages for raw carrier phase data and a measure of data reliability for reliability assessment.